In the light of environmental problems, the recent mainstream of hardening compositions used for forming a solder resist of a printed-wiring board is a composition using a photo-setting or thermosetting resin that can be developed with a diluted aqueous alkaline solution. A generally used resin that can be developed with a diluted aqueous alkaline solution is a hardening resin obtained by adding an acid anhydride to a reaction product of an epoxy resin and a monocarboxylic acid containing an unsaturated group.
For example, Patent Document 1 discloses a hardening resin obtained by adding an acid anhydride to a reaction product of a novolac-type epoxy resin and a monocarboxylic acid containing an unsaturated group. Patent Document 2 discloses a hardening resin obtained by a process including: the reaction of an aromatic epoxy resin having two glycidyl groups in one molecule with an aromatic alcohol resin having two phenolic hydroxyl groups to obtain a secondary alcoholic hydroxyl group; the reaction of this hydroxyl group with epihalohydrin; and the addition of a monocarboxylic acid containing an unsaturated group to the reaction product of the previous step, followed by the addition of an acid anhydride.
As just described, several kinds of hardening resins have been proposed to date, which are nowadays widely used for actual production of printed-wiring boards.
The density of the printed-wiring boards has been increased with the recent decrease in the size and weight of electronic devices, and this increase in the density of the printed-wiring boards has increased the demand for a solder resist with higher performances. However, the conventionally available commercial hardening compositions do not have adequate development properties for the production of high-density printed-wiring boards; the image resolution achieved by those compositions is insufficient for the required high density. Furthermore, a high-density printed-wiring board produced by using a solder resist made of a conventional hardening composition has the problems of the decrease in the flexibility as well as the decrease in the electrical insulation property and the bleaching under high temperature and humidity.
As a hardening composition for solving the flexibility problem, a photosensitive carboxyl prepolymer has been proposed (refer to Patent Document 3), which is obtained as follows: An alcoholic secondary hydroxyl group on a bisphenol A-type epoxy resin is made to react with epihalohydrin to obtain a polyfunctional epoxy resin. Subsequently, this epoxy resin is made to react with 0.2-1.2 mol of an ethylenically unsaturated carboxylic acid per epoxy equivalent weight, and further with 0.2-1.0 mel of one or both of a polybasic carboxylic acid and its anhydride per epoxy equivalent weight.
Patent Document 4 discloses a photosensitive prepolymer having a carboxyl group. This prepolymer is obtained as follows: A mixture of a novolac-type epoxy resin and a rubber-modified bisphenol A-type epoxy resin is made to react with 0.2-1.2 mol of an ethylenically unsaturated carboxylic acid per epoxy equivalent weight of the aforementioned mixture, and the product of this reaction is further made to react with 0.2-1.0 mol of a polybasic carboxylic acid and/or its anhydride per epoxy equivalent weight of the aforementioned mixture.
Patent Document 5 discloses a hardening composition using a vinyl ester resin, where the vinyl ester resin is obtained as follows: An epoxy resin having two or more glycidyl groups in one molecule is made to react with an ethylenically unsaturated monocarboxy lie acid and a polybasic acid at a predetermined ratio, after which a portion of the primary and/or secondary hydroxyl groups in the product of the previous reaction is subjected to an esterification reaction with a chloride of an ethylenically unsaturated monocarboxylic acid, and then a portion or all of the hydroxyl groups remaining in the obtained reaction product is subjected to an esterification reaction with a polybasic acid and/or a anhydride of a polybasic acid.
A resist film obtained by using the aforementioned hardening composition is excellent in flexibility. However, when the reaction ratio of the ethylenically unsaturated carboxylic acid is within a certain range, a gel or high polymer is formed during the reaction, so that no sufficient development property can be obtained. For example, when 0.2-0.7 mol of an ethylenically unsaturated carboxylic acid per epoxy equivalent weight is made to react with the aforementioned epoxy resin, and the resultant epoxy resin containing an ethylenically unsaturated carboxylic group is made to react with 0.2-1.0 mol of a polybasic carboxylic acid and/or its anhydride per epoxy equivalent weight, then the polybasic carboxylic acid and/or its anhydride acts as a crosslinking agent for the aforementioned epoxy resin containing an ethylenically unsaturated carboxylic group, causing the resin to turn into a gel or high polymer and thereby lowering its development property. Thus, the aforementioned hardening composition does not help in producing high-density printed-wiring boards required for supporting the recent decrease in the size and weight of electronic devices.
On the other hand, a method for improving the tack-free property of a hardening resin by utilizing the formation of the high polymer has been proposed (refer to Patent Document 5). The tack-free property is one of important properties of a photoimageable liquid resist. However, it is incompatible with the alkali development property. Given this problem, a technique disclosed in Patent Document 5 is aimed at achieving a trade-off between the tack-free property and development property of a hardening resin by the addition reaction of a polybasic acid and/or an anhydride of a polybasic acid to a hydroxyl group. However, even this technique cannot achieve adequate development property for the production of high-density printed-wiring boards required for supporting the recent decrease in the size and weight of electronic devices.
There are more hardening compositions proposed to solve not only the flexibility problem but also the problem of electrical insulation property, such as a hardening composition using a photosensitive carboxyl resin obtained by the reaction of a novolac-type phenol resin with alkylene oxide, followed by the reaction of the obtained product with a polybasic acid anhydride (refer to Patent Document 6), or a hardening composition using a photosensitive carboxyl resin obtained by the reaction of a novolac-type phenol resin with either alkylene oxide or cyclic carbonate, followed by the reaction of the obtained product with a monocarboxylic acid containing an unsaturated group and a saturated aliphatic monocarboxylic acid and/or an aromatic monocarboxylic acid, and the reaction of the obtained product with a polybasic acid anhydride (refer to Patent Document 7). However, the development property of these compositions is rather low. Although a high-density printed-wiring board produced by using a resist made from those compositions is has an excellent electrical insulation property, it has a low level of hardness and also undergoes whitening.
Patent Document 1: Japanese Unexamined Patent Application Publication No. S61-243869
Patent Document 2: Japanese Unexamined Patent Application Publication No. H5-32746
Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-65117
Patent Document 4: Japanese Unexamined Patent Application Publication No. H9-5997
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-067815
Patent Document 6: International Publication No. WO 02/024774 A1
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2005-91783